<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box {
      width: 200px;
      height: 200px;
      background-color: pink;
    }
  </style>
</head>
<body>
  <div id="app">
    <!-- <div class="box" :style="{ width: '400px', height: '400px', 'background-color': 'green' }"></div> -->
     
    <div class="box" :style="{ width: '400px', height: '400px', backgroundColor: 'green' }"></div>
  </div>

  <script src="https://cdn.jsdelivr.net/npm/vue@2.7.16/dist/vue.js"></script>
  <script>
    new Vue({
      el: '#app',
      data: {
        
      }
    })
  </script>
</body>
</html>